3 Answers
3

Rabbi Achai Gaon, in Shiltos (165) states that non-Jews are obligated to marry and have children.

See also Rabbi Moshe Feinstein in Igros Moshe (E"H book 4 ch. 29) where he concludes that the obligation of Pru Urvu (be fruitful and multiply) is an obligation on all of mankind, as a communal commandment. This also seems to be the opinion of Drisha (E"H ch. 1 note 2). It is the individual mandate to have children that was only given to Jews.

See also Yevamos (62a) where apparently Rabbi Yochanan held that one fulfills his obligation with non-Jewish children he had before his conversion to Judaism. (Rambam however adds that those children must have also converted.)

(UPDATE: on reflection, since non-Jews do not have the ability to effect a legally binding Kidushin/Nisuin, they couldn't be obligated to marry, at least not in the same sense as a Jew.

Noahide-law adultery involves not a married woman, but a woman who is living together with a man without the formal institution of marriage, aka "be'ulas ba'al".)

The position of R' Moshe that you quote doesn't seem to lead to a mandate to get married, since it's not an individual mandate, and anyway, one needn't necessarily get married in order to have children (unless non-marital relations are prohibited under Noahide immorality, which I doubt).
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Isaac MosesApr 22 '10 at 20:57

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Same is true of R' Achai's position. His wording is: דמחייבין דבית ישראל למינסב נשי ואולודי בני ומיעסק בפריה ורביה... אפילו גוים מפקדי אפריה ורביה. So he seems to be drawing a clear distinction: Jews have a dual obligation, to marry and to have children, but non-Jews have an obligation only to do the latter. The implication, then, is that they needn't marry for this purpose; having children out of wedlock would satisfy their basic halachic obligation.
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AlexApr 22 '10 at 22:12

I agree with both of you, but see my update, above.
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BarryApr 26 '10 at 14:29

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I don't know if your edit is correct. Rambam (Hil. Issurei Bi'ah 14:21) states that halachah does recognize "ishus" (a marital status) between non-Jews. It is true, based on the words "be'ulas baal," though, that this doesn't go into effect until they actually cohabit (Rambam, Hil. Melachim 9:7), unlike in a Jewish marriage where this state is entered upon at kiddushin.
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AlexApr 26 '10 at 18:29

Apparently not. Rambam (Laws of Marriage 1:1,4) states that Torah law (for Jews) requires a marriage to be effected in the presence of witnesses, and outlaws non-marital sex. But neither of these applies to non-Jews.

Nor would they be required to marry (or have a non-marital sexual relationship) in order to have children. The Gemara (Sanhedrin 59b) states that the mitzvah of "be fruitful and multiply" (Gen. 1:28) is one of the ones that was not repeated at Sinai, meaning that only Jews are obligated in it.

(To be precise, the Gemara says that it was repeated - in Deut. 5:26, by implication - but that this was done in order to teach us how halachic enactments have to be repealed, rather than to make it binding on non-Jews. Presumably, the She'iltos (cited in Barry Frank's answer) would understand this Gemara differently: this mitzvah is repeated for two reasons - to make it obligatory on non-Jews as well as to teach us about the rules of halachic repeal.)

Just the opposite: there are meforshim who understand that very Rambam to indicate that non-Jews are required to designate a woman as a wife (for purposes of establishing adultery) and the only difference is that Jews are required to have witnesses, whereas non-Jews do not. See: hebrewbooks.org/rambam.aspx?mfid=19161&rid=2853 for one example although many of the meforshim understand it that way.
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CuriouserDec 15 '11 at 0:46

@Curiouser: thanks for that reference, but I'm not entirely sure it's relevant to the question being asked here. It's a good answer for judaism.stackexchange.com/questions/2718/…, but here the question was whether non-Jews are obligated in the mitzvos that Rambam associates with Hilchos Ishus (to marry, not to have relations outside marriage, and to have children) - and that source doesn't directly bear on any of those.
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AlexDec 15 '11 at 3:38

Maybe you're right -- on the other hand the question here is pretty general.
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CuriouserDec 15 '11 at 15:39

It would seem that there is only one positive obligation among the Sheva Mitzvos B'nei No'ach, which is to carry out justice. These 7, from the standpoint of the Torah, are the only legal obligations on non-Jews. Since this obligation is unrelated to marriage, I would conclude the answer is no.

It seems from the Wikipedia article you pointed to that according to some opinions, there are more obligations on gentiles than seven. Levirate marriage is mentioned specifically by R' Saadia Gaon. R' Lichtenstein apparently counts 66! So, it's not so simple.
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Isaac MosesApr 20 '10 at 21:18

@Isaac Moses, in the ensuing months it has come to my attention just how wrong my assumptions here were. It turns out many people include many more things in the obligations of B'nei No'ach even though only those 7 are listed under the title "mitzvos B'nei No'ach" in Sanhedrin. An amazing example is found in אגרות משה א"ח חלק ב סימן כו.
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WAFDec 2 '10 at 3:35

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WAF, when the commandment to get married was given in Genesis 1:28, it was meant to all Mankind, Jews and non-Jews. BTW, Jews had not even risen. So, in reply to the op-question, the answer is yes. IMO, that commandment does carry out justice, between G-d and man, as well as between a man and his woman.
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Ben MasadaDec 17 '10 at 18:12